Sarah Brown, playing Claudia, actually created the role of Carly in 1996. She played her until 2001, winning three Daytime Emmys before leaving the show.
Laura Wright took over the role in 2005 to become Carly No. 4) and has played her ever since.
Now comes Splainin Time!
Port Charles, New York, is one of those towns in Toobworld which serve as experimental sites by researchers from the Future. They use 'Quantum Leap' technology to take the place of residents in these towns without anyone around them being the wiser. Sometimes the original residents are returned; sometimes they are replaced several times over by other researchers.
Carly Benson was one of those abducted and taken into the Future. Several times over she was replaced by Time Travelers who used specially designed auras so that they continued to look like her.
As for Claudia looking like the original Carly, I could have gone the safe and practical "identical twin separated at birth" route. It would have been accepted in the soap opera milieu. But that would create continuity problems with their birth mother's history. There was also the tradition of "identical cousins" begun with 'The Patty Duke Show'. But again, eventually that would have to be addressed within the show.
This isn't the town of Harmony, as seen in 'Passions'; it's Port Charles. So magical transformations should not be considered. And I don't think the concept of cloning was really that believable on 'Guiding Light', so I don't think it should be attempted here.
But this splainin should do the trick - plastic surgery. We never met Claudia Zacchara before she arrived in Port Charles, so it's possible she did not always look the way that she does now. It was done far enough in the past that it's no longer even mentioned by members of her own family, like mob boss dad Anthony Zacchara.
Coincidence plays a major part in Toobworld, and it could be a coincidence that she chose a look that was well-known in her new hometown. But I don't think that's the case. I think she deliberately chose it in order to better insure her "chance encounter" with Sonny Corinthos.
When they "accidentally" met in the bar and spent the night together without even knowing each other's name, she didn't need the information. She already knew; she arranged the meeting. And she chose her new look in order to maximize his attraction to her. There must be enough differences in her look so that she doesn't look exactly like Carly, but we just can't notice them from our vantage point in the Trueniverse.
However, she still looked enough like Carly so that subconsciously, Sonny was drawn to her. Later, at the mob meeting she organized where they met "officially", she acted surprised to find out that the guy she slept with was from a rival mob family.
This splainin about her plastic surgery being just slightly different from the look of the original Carly would then be the reason why the new Carly didn't realize that it was like looking in a mirror when she finally met Claudia.
Whew! That was more complicated than I thought! But I'm sticking with it......